1. Field of Invention
This invention relates to a pulse width control IC circuit, used in electronic equipment, such as computers; and more particularly, to a pulse width modulation (PWM) IC and switching elements in an auxiliary power supply, and to a switching power supply unit using such auxiliary power supply, wherein the number of components on the primary side is reduced when the pulse width control IC circuit is incorporated therein.
2. Description of the Prior Art
As disclosed in Japan Unexamined UM Appl.1990/37592, the conventional switching power unit combines a main converter and an auxiliary converter. The main converter has an output voltage regulating circuit and the auxiliary converter has a simple circuit configuration, such as a ringing choke converter (RCC power supply). The capacity of the main converter is several hundred watts, while the capacity of the auxiliary convert is several watts.
However, even an auxiliary converter is an independent converter and thus requires switching elements and control elements, as well as a number of components equal to that of the main converter, even though such auxiliary converter consumes only a few watts. Accordingly, the ratio of cost of the primary circuit and the packaging area on the printed circuit board in the auxiliary converter to the cost of an entire power supply unit is great. In addition, since a high breakdown voltage is required for switching elements for the auxiliary converter, large package FETs and transistors having larger current ratings than were actually necessary were employed, thereby requiring countermeasures in order to reduce costs.